Bisphenol-A has been an extremely useful chemical for many decades. As a difunctional monomer it has been used in the preparation of numerous polymers. For example bisphenol-A (2,2-bishydroxyphenyl-propane) has been utilized in preparing inter alia, epoxy resins, polyetherimides, polyarylates and, in particular, polycarbonates. In certain of these polymer systems, particularly the epoxy systems, the purity of the bisphenol-A employed in the polymer reaction need not be that high. Epoxy resins only need bisphenol-A of approximately 95% purity. The impurity which is present in the greatest amount in such systems is generally orthopara bisphenol-A. However with other polymer systems, particularly polycarbonate, the purity of the bisphenol-A must be substantially higher. Purities of bisphenol-A of about 99.50% or higher preferably 99.80 or 99.90% or higher are desirable and in many cases necessary for the preparation of bisphenol-A polycarbonate. Therefore there has been substantial attention directed to the preparation and purification of bisphenol-A.
The art is replete with references directed to the preparation of bisphenol-A. Usually this is done by the condensation of phenol with acetone in the presence of a catalyst system. Generally the catalyst is an acidic catalyst. For many years one of the particularly useful catalyst systems in the patent art and employed commercially was hydrochloric acid. Although the economics of the process are initially good with respect to conversion of the reactants to bisphenol-A, the maintenance of the apparatus is costly. The hydrochloric acid is extremely corrosive and ordinary metallic reactors and piping must be changed on a frequent basis. Obviously glass lined reactors or certain alloyed metals can be employed, however these are quite expensive. In later years there seems to be the tendency to use a heterogeneous acidic catalyst system wherein the acidic catalyzation occurs at the catalyst surface and is actually bound to the catalyst. In this manner the "acid" does not flow with the unused reactants and bisphenol-A. Such catalyst systems are generally sulfonated polystyrenes which are substantially crosslinked such as the Amberlites and like materials. After the bisphenol-A is prepared, various isolation and purification procedures are known. Many of these appear in the relatively voluminous patent art. Generally phenol is distilled off to a great extent and/or the initial purification done by adduct crystallization of the bisphenol-A phenol adduct. Distillation of the bisphenol-A itself can also be employed. The purification of the bisphenol-A can then be accomplished through the addition of various organic solvents such as toluene or methylene chloride so as to remove the bisphenol-A from various impurities. Additionally water and various glycols such as ethylene glycol and glycerin have been used alone or together to purify the bisphenol-A from its impurities.
As stated previously the preparation of high purity bisphenol-A is extremely important in the preparation of polycarbonate. For handling purposes, storage purposes and general purity reasons, the bisphenol-A has been isolated in the orthorhombic crystal or needle (dendritic) forms, orthorhombic preferred. Until the present time, the methods of obtaining an orthorhombic crystalline form have required the use of additional water or glycols. Examples of such glycols are ethylene glycol, glycerine, butanediol and water cosolvent systems such as in U.S. Pat. No. 4,113,974. Utilization of other solvents to obtain crystalline bisphenol-A such as toluene or methylene chloride have resulted in the obtaining of the needle (dendritic) form. Each of these processes utilize such additional chemicals as organic solvents and/or water and require the utilization of such procedures as extraction, slurry handling, centrifugation, filtration, vacuum systems, distillations, dryers and waste water treatment facilities.
A new method for purifying bisphenol-A has been discovered. None of the procedures mentioned in the last sentence of the previous paragraph need be used. This process is capable of obtaining extremely high purity bisphenol-A in a highly economic, efficient manner. The new method involves purifying impure bisphenol-A by fractional melt crystallization in a falling film dynamic crystallizer. In this manner highly pure bisphenol-A can be obtained without contamination by extraneous solvents or other materials. These results were extremely surprising since there is nothing in the art to lead one to believe that this process would work to purify bisphenol-A to such high purity levels, particularly in view of the potential contamination from degradation reactions as well as back mixing of impurities. Additionally this process is so efficient that high purity bisphenol-A can be prepared in an economically practical number of stages.
Additionally this process has the surprising aspect of freezing out the bisphenol-A in its orthorhombic crystalline form. Therefore this method produces orthorhombic crystalline bisphenol-A without the presence of additional solvents such as water, glycol or mixtures thereof. The process is extremely efficient and productive. Starting with crude bisphenol-A obtained from a reaction which prepares the bisphenol-A, high purity bisphenol-A in high yield can be prepared therefrom without the use of any or all of the following: organic solvents, water extraction, slurry handling, centrifugation, filters, vacuum systems, dryers or distillations.